Patients with heart failure (HF) account for over 1,200,000 VA outpatient visits per year, and HF remains the most common cause for hospital admission in the VA. Approximately 1/3 of veterans with HF have `preserved' ejection fraction (HFpEF), or relatively normal contractile function of the heart; such patients suffer functional decline and poor quality of life, and half die within 5 years after diagnosis. Risk factors for developing HFpEF are more common in veterans than the general population, and the burden of HFpEF to the VA system will rise in the years ahead as these veterans age. Preventive efforts are critical, but are hampered by gaps in knowledge related to HFpEF pathophysiology. The long term goal of this proposal is to prevent the onset of HFpEF in at-risk veterans. Hypertension (HTN) confers the highest population-attributable risk for HFpEF, particularly when accompanied by the metabolic syndrome, a constellation of obesity, insulin resistance, and dyslipidemia. Animal models of HTN and metabolic syndrome develop HFpEF due to microvascular oxidative stress and inflammation induced by high sodium intake. Recent data from cardiac biopsies confirm similar mechanisms in human HFpEF. Dietary sodium restriction is widely recommended to prevent HTN-associated heart disease in humans, but this advice is now controversial. Few studies have examined how individual differences in response to sodium intake affect risk. ?Salt-sensitive? persons have blood pressure (BP) that changes in parallel with sodium intake, and commonly develop cardiovascular abnormalities associated with HFpEF. The overall objective of this proposal is to evaluate salt-sensitivity as a novel, diet-responsive risk factor for incident HFpEF in veterans with HTN and metabolic syndrome. The central hypothesis is that the sodium-restricted Dietary Approaches to Stop Hypertension (DASH/SRD) eating pattern will improve cardiovascular functional and structural risk factors for HFpEF in veterans with the salt-sensitive phenotype. Guided by findings in experimental models, cohort studies, and strong preliminary evidence from our research group, this hypothesis will be tested in a two-phase study and by pursuing three specific aims: 1) Determine effects of DASH/SRD on functional and structural cardiovascular HFpEF risk factors in salt-sensitive vs. salt- resistant veterans, 2) measure the effect of an electronically-delivered tailored-messaging intervention on DASH/SRD adherence, and 3) determine effects of DASH/SRD intervention and adoption on microvascular function and assess the endothelial glycocalyx as a biomarker of cardiovascular response to DASH/SRD. Phase 1 of the study is a crossover-randomized comparison of DASH/SRD vs. control diet for two weeks each, and Phase 2 a 6-month extension to promote DASH/SRD adherence. The salt-sensitive phenotype will be defined by between-diet changes in 24-hour mean BP during Phase 1. In Phase 2, the efficacy of motivational interviewing-based counseling and the Women's and Men's Hypertension Experiences and Emerging Lifestyles Intervention (WHEELS-I), a tailored messaging program, to sustain DASH/SRD adherence, will be compared. Echocardiography and arterial tonometry will be used to assess HFpEF-related cardiovascular parameters during short- and longer-term dietary modification and their interaction with salt-sensitivity. In vivo microscopy and novel blood testing will assess microvascular function and the integrity of the endothelial glycocalyx, a blood vessel lining that is sodium-responsive and may mediate the adverse effects of salt- sensitivity. This proposal is innovative because it represents the first study to examine salt-sensitivity as a factor promoting HFpEF in veterans with HTN and metabolic syndrome, the highest risk group for incident HFpEF. Moreover, it aims to link microvascular dysfunction, an important pathway in human HFpEF, with endothelial glycocalyx damage, a potential biomarker for sodium-mediated vascular risk. The proposed research is significant because it will vertically advance our understanding of how dietary factors contribute to the pathophysiology of HFpEF, a major and growing health threat to veterans.